1. Field of the Invention
The present invention relates to a control system for a heat fixing apparatus used in a copying machine or a printer. Particularly, it relates to a control system for a fixing apparatus used in a super-high speed copying machine or a printer, provided with a heating means only in one roller.
2. Description of the Related Arts
It is well-known to apply a heat fixing apparatus using in a copying machine or the like for fixing an image printed by toner.
Generally in this type of a heat fixing apparatus, a heater roller having a built-in heater is in contact with a press roller and rotates together with the latter, whereby a paper printed by toner passes between these rollers so that toner can be fixed on the paper. In this case, it is important for achieving stable fixing that the temperatures of the heater roller and the press roller must be maintained as closer as possible to a predetermined value.
It is publicly known that the heater roller and the press roller are rotated together for a predetermined time, while being in contact with each other, to uniformly warm up the press roller with heat generated at the heater roller (see, for example, Japanese Examined Patent Publication (Kokoku) No. 61-31462).
Also, it has been proposed that the heater roller start rotating while in contact with the press roller at a first target temperature that is lower than a predetermined standby temperature and stop rotating at a second target temperature that is higher than the first target temperature (see, for example, Japanese Examined Patent Publication (Kokoku) No. 61-31463).
In these prior arts, however, in the case that the apparatus stops for a long period after the warming-up operation or that a copying interval becomes longer, the temperature of the heater roller lowers significantly and inferior fixing occurs, because the temperature of the press roller is lowered considerably during the standby period, whereby a large amount of heat is transferred to the press roller from the heater roller at the initial stage of the copying process.
In a low speed copying machine, even if a temperature drop in the heater roller occurs, there is sufficient time to recover the temperature and no fixing problems may occur. On the contrary, in a high speed copying apparatus having a capacity of, for example, 100 sheets per minute, several sheets of paper may pass through a fixing apparatus before the temperature reaches the predetermined temperature, whereby the temperature drop in the heater roller has a significant influence.
FIG. 11 is a chart for explaining the above problems inherent in the prior art copying machine, in which the abscissa and ordinate represent time and the surface temperature of rollers, respectively.
That is, in the case that a surface temperature TP of the press roller is at greatly low value, a surface temperature TH of the heater roller is lowered once from TH.sub.1 to TH.sub.2 during a period between time t.sub.1 and t.sub.2, and the surface temperature TP of the press roller rises from TP.sub.1 to TP.sub.2.
When the surface temperature TH of the heater roller is lowered, a heater is energized to warm-up the heater roller, but if a silicone rubber having a lower thermal conductivity is used as a surface layer of the heater roller, the surface temperature TH reaches an optimal temperature TH.sub.3 at a delayed time t.sub.3.
In a high speed copying machine, since several sheets of paper may pass through the rollers during a period between t.sub.2 and t.sub.3, the occurrence of inferior fixing cannot be prevented.
A hard roller having a surface layer member of fluorine resin (such as TEFLON.RTM.) has been used in the heat fixing apparatus, however, this roller has a drawback in that an image obtained after the fixing process becomes too glossy to be favored. Therefore, a soft roller using a silicone rubber as a surface layer member is favorable from the view point of an improvement in image quality. Since silicone rubber has a lower thermal conductivity, however, the temperature controlability of the heater roller and the press roller is worsened. In addition, since the heat durability thereof is insufficient, the temperature of the heater roller cannot be raised to a higher level.
To solve the above problems in the prior arts, it is proposed that a heater be provided not only in the heater roller but also in the press roller, and both rollers be maintained at substantially the same temperature (see, for example, Japanese Examined Patent Publication (Kokoku) No. 2-10427).
The fixing apparatus in which two heaters are provided in both the rollers can be suitably applied to a high speed copying machine, but is expensive due to the provision of two heaters, and also has a large power consumption.